Big bang birthday: Six mysteries of a cosmic bombshell

In 1964, a pair of engineers at Bell Labs in New Jersey tried to build a better antenna and ended up uncovering the origins of the universe. After ruling out city noise, nuclear bombs and pigeon poop, Arno Penzias and Robert Wilson argued that a strange radio hiss in their readings was the first confirmed signal of the cosmic microwave background (CMB). This relic glow emerged as a result of the big bang and now permeates the universe.

The discovery solidified big bang theory as our best explanation for cosmic origins, and Penzias and Wilson went on to net a Nobel prize. Now, 50 years later, the CMB (pictured above) has helped us figure out the universe's age, shape and components, as well as details about how it has evolved. But with almost every discovery, the CMB raised new and more vexing questions. Here are six of the biggest lingering mysteries sparked by studies of the big bang.

1. Why is the early universe so smooth?

At first, maps of the CMB looked too good to be true. After the big bang, matter should have flown apart and formed random clumps. But the CMB showed that the universe was incredibly uniform, as if far-flung regions had somehow stayed in contact during the universe's early expansion.

In the 1980s, physicists came up with the idea that the universe went through a period shortly after the big bang when it ballooned exponentially – a theory now called inflation.

Recent maps of the CMB have borne out some predictions of this model, but not all of them. Even if the theory is true, we still have no idea what caused inflation, when it started and why it stopped. We could have an answer soon. Inflation should have triggered ripples in space-time called gravitational waves, and signatures of these waves should show up in upcoming, higher-resolution maps of the CMB, says Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "It's not easy by other means to develop gravitational waves of the type one expects," he says.

2. Was there anything before the big bang?

Inflation erased any record of what came before the rapid expansion, so we can't answer this question by looking at the sky. In addition, the universe started out so hot and dense that the equations we use to describe its expansion and evolution break down.

A theory that unites these equations with quantum mechanics could make mathematical predictions of what, if anything, existed before the big bang, but such a theory remains elusive. Of course, that has not prevented people from using the CMB to speculate about bouncing universes that cycle through endless bangs and crunches, or multiverses that bud off from a single original universe.

3. Could ancient life have emerged in the big bang's glow?

The CMB's light comes from superheated gas, or plasma, that filled the early universe. This matter cooled with time to give rise to stars and galaxies, and today space is too frigid to host life as we know it on worlds far from their stars. But temperature readings of the CMB hint that a mere 15 million years after the big bang, the glow would have been warm enough to make the whole universe one large life-friendly zone. This epoch would have lasted a few million years, enough time for microbes to emerge but not complex life, Loeb suggests.

4. What are dark matter and dark energy?

Astronomers already knew in 1964 that some parts of the universe had more matter than we could see: based on our best understanding of gravity, galaxies whip around in clusters and spin faster on their axes than their visible matter should allow them to without flying apart. The CMB showed us that this unseen dark matter makes up about 80 per cent of all matter in the universe. But we still don't know what the mysterious stuff is made of, or whether it will ever show itself in space-based experiments or underground detectors.

Also, supernova studies revealed in 1998 that the universe has not only been expanding since the big bang, but also flying apart at an accelerating rate. This effect is attributed to a strange force called dark energy, and the CMB revealed that it makes up 68 per cent of everything in the universe. Beyond that, dark energy remains one of the most mysterious forces in physics.

5. What is the universe's ultimate fate?

Studies of the CMB could help us trace dark energy's behaviour over time, ultimately telling us what might happen when the universe ends. If dark energy steadily increases in strength, then the universe may be doomed to tear itself apart in a big rip. If it increases and then decreases again, however, some structures could grow again from the ashes of a dying universe. And if dark energy holds steady at the rate we see today, the universe will expand forever and end up dark and cold.

6. Will the big bang become an untestable theory?

If a steady expansion continues long enough, a single wavelength of the CMB will be stretched until it is as large as the universe itself. That means any humans still around in a trillion years or so won't be able to detect the CMB at all, says Loeb. "The sense that we have of cosmological studies will be lost, because we won't be able to find any trace of the big bang on the sky," he says. "In the future, we will have a story of the big bang, and we won't be able to verify it. So will cosmology turn into a religion? At least for a trillion years, we won't have to worry about that."

For more big bang excitement, watch Wilson and Loeb speak during a 50th anniversary celebration at the Harvard-Smithsonian Center for Astrophysics at 1930 EST today (0030 GMT 21 February). More info here, or simply watch it here.

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